Spatial and Temporal Resolution Requirements for Real-Time Temperature Measurements in Perennial Crops
Eileen M. Perry1, Pedro Andrade1, Jose Chavez2 and Francis Pierce1,
(1)Washington State University, Irrigated Center for Precision Agricultural Systems, 24106 N. Bunn Road, Prosser, WA 99350, USA, (2) Conservation & Production Research Lab. USDA-ARS, 2300 Experiment Station Road, Bushland, TX 79012, USA
Presented at the ASA-CSSA-SSSA International Annual Meetings (November 12-16, 2006)
Abstract:
Growers of high value perennial crops must make management decisions based on conditions that can vary on spatial and temporal scales of meters and minutes. For example, tree fruit growers expend tremendous effort and cost to protect against frost damage to sensitive flowers and buds as critical changes in temperature can occur within minutes. While 15 minute averages of weather station data may be adequate for some management issues such as irrigation scheduling, other management issues such as temperature monitoring for frost protection require data with high spatial and temporal resolution. This study evaluated small but critical temperature changes during frost events of April 2006 in an apple orchard (Malus domestica Borkh. cv. 'Granny Smith where reflective mulch ground cover was installed to benefit fruit growth and maturation. Multiple temperature sensors were installed in rows with and without the reflective mulch to obtain high spatial and temporal resolution data. Air temperature was measured at one minute intervals for several heights in the canopy as well as for the blossom temperatures and shallow soil temperature. Temperature differences between the two ground cover treatments for the same height changed throughout a 24 hour period. Flower temperature differences between the treatments fluctuated, although flower temperatures were consistently colder in the mulch site during freezing conditions. Averaging one minute temperature measurements to a 15 minute interval reduced or eliminated the treatment differences. The results demonstrate that in situ measurements can be important for extreme events (such as frost protection). Temperature fluctuations can occur over short periods that go undetected when measurements are made outside the actual canopy environment, and/or after averaging over longer periods such as the traditional 15 minute intervals. During these frost events, the grower was unaware that the flower temperatures were up to 1.5 C colder for the mulch site, which could have produced damage and hence impacted fruit development.
Figure 1. Pre-dawn thermal infrared imaging reveals difference between surfaces with and without reflective mulch installed. The sites labeled 3 and 4 were later instrumented to measure real-time temperature at 1 minute intervals.
Figure 2. Sensors installed for real-time temperature measurements, on east side of N-S rows. Two sites were selected to represent conditions with and without the reflective mulch.
Figure 3. Eight co-located thermistors measured air temperature over a 48 hour period. Data was collected every 10 seconds and averaged for 1 minute intervals. Mean of the temperature values from the 8 sensors were averaged for each minute, and the standard deviation of the sensors from this mean value is plotted against time. Based on standard deviations, temperature differences between sensors greater than 0.2 C represent true differences in the air temperature.
Figure 4. Differences in air temperature between sites with and without reflective mulch. Positive values indicate warmer temperatures at the mulch site.
Figure 5. Difference in temperature between blossoms at sites with and without the reflective mulch. Positive values indicate warmer temperatures at the mulch site. The one minute data, shown as the solid lines, was averaged for 15 minute intervals to demonstrate the information that is lost. Note that the thermistors were installed in the blossoms without shielding, so the daytime temperatures are not accurate and were deleted from graph.
Methods and Data:
Differences in surface temperatures ranging from 1.2 – 2.0 C between areas with and without reflective mulch installed were first seen in thermal infrared remote sensing (Fig. 1) in early April 2006. To validate these apparent differences in temperature, multiple temperature sensors were installed in an apple orchard (Malus domestica Borkh. cv. 'Granny Smith') in rows with and without the reflective mulch to obtain high spatial and temporal resolution data (Fig. 2). A cross-sensor validation test determined that temperature differences between sensors greater than 0.2 C represent true differences in the environmental temperature (Fig. 3). Air temperature was measured at one minute intervals for several heights in the canopy as well as for the blossom temperatures and shallow soil temperature.
Results
Temperature differences between the two ground cover treatments for the same height changed throughout a 24 hour period (Fig. 4). Flower temperature differences between the treatments fluctuated, although flower temperatures were consistently colder in the mulch site during freezing conditions (Fig. 5). Averaging one minute temperature measurements to a 15 minute interval reduced or eliminated the treatment differences (Fig. 5). An energy balance was also calculated to validate the observed temperature differences, using air temperature and relative humidity from the in-situ sensors, surface temperature from the thermal infrared imagery, and other meteorological measurements from a local weather station. The pre-dawn net energy balance values were larger, negative numbers for the site with the mulch installed, indicating a net energy loss away from the surface (Fig. 6). The imagery, measurements and energy balance results demonstrate that in situ measurements can be important for extreme events (such as frost protection). Temperature fluctuations can occur over short periods that go undetected when measurements are made outside the actual canopy environment, and/or after averaging over longer periods such as the traditional 15 minute intervals.
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